Solar Smoke Rings

February 3, 2000 -- In J.R.R. Tolkien's well-known
fantasy The Hobbit, the diminutive inhabitants of the
Shire loved to smoke pipe-weed and blow intricate rings to delight
their visitors. Only the powerful wizard Gandalf could best a
Hobbit in that difficult art.

Here in the real world, the best way to enjoy an enchanting tobacco-free
smoke ring may surprise you. Simply tune in to the Solar and
Heliospheric Observatory (SOHO), an Earth-orbiting observatory
with an eye on the biggest puffer in the solar system -- the
Sun.

Beginning on January 28, 2000, SOHO recorded a series of dramatic
solar coronal mass ejections (CMEs) as the Sun belched billions
of tons of hot gas into interplanetary space. One of them, sighted
on January 31, displayed swirling loops reminiscent of Bilbo
Baggins's most elegant smoke rings.

Above: These coronagraph images captured
by SOHO on January
31, 2000, show a beautiful coronal mass ejection erupting from
the southwest limb of the Sun. The red-colored animation, which
targets the Sun's inner corona from 1.1 to 3 solar radii, spans
a six hour period beginning at 0154 UT. The swirls inside the
CME are about 30 times bigger than Earth! The blue image displays
the outer corona from 1.5 to 30 solar radii. It shows the CME
at 1154 UT, about 10 hours after the eruption began. For more
information about SOHO coronagraphs, click
here.

Coronal mass ejections can carry up to 10 billion tons
of plasma traveling at speeds as high as 2000 km/s. When they
collide directly with Earth they can excite geomagnetic storms,
which have been linked to satellite communication failures. In
extreme cases, such storms can induce electric currents in the
Earth and oceans that can interfere with or even damage electric
power transmission equipment.

Fortunately, none of the latest CMEs were headed in our direction.
Earth-directed mass ejections produce what astronomers call "Halo events."
As they loom larger and larger they appear to envelope the Sun
itself.

The recent batch of CMEs -- mostly seen in profile -- look very
much like expanding tangled loops. While there is some visual
similarity to smoke rings, the physics of a CME is different.
Common down-to-Earth smoke
rings are spinning doughnut-shaped bundles of unmagnetized,
neutral gas. They have lower pressure on the inside than the
outside, and can travel considerable distances as they expand
and dissipate into the surrounding air. On the other hand, gas
inside a coronal mass ejection is completely ionized and permeated
by strong magnetic fields. The magnetic forces dominate the movements
and structure of the plasma. Most of the loops seen in a CME
are hot glowing gas trapped inside curved magnetic fields.

If not for the coronagraphs
on the Solar and Heliospheric Observatory, many CMEs would never
be noticed. At present, SOHO is only spacecraft that monitors
the Sun's outer corona nearly around the clock. Two others, TRACE and Yohkoh,
keep an eye on the inner corona, but CMEs are harder to spot
there. Space weather forecasters rely on SOHO for early warnings
of many Earth-directed solar eruptions, a service that is increasingly
important as we approach the solar maximum in mid-2000. (click for animation)

"During solar maximum we often have more than one coronal
mass ejection every day," explains David Hathaway, a solar
physicist at the NASA Marshall Space Flight Center. "The
basic cause of CMEs is fairly well understood. Like solar
flares, they occur whenever there's a rapid, large-scale
change in the sun's magnetic field. Solar flares and CMEs often
occur together, as they did this weekend, but not necessarily
because the flare triggers the CME or vice versa. One can happen
without the other and frequently during solar maximum we see
CMEs without an associated flare."

The CME on January 28, pictured here,
was associated with a weak C1.1-class solar flare near sunspot
group 8848. That active region had 3 tiny sunspots and a relatively
uncomplicated beta-type
magnetic field structure. C-class flares are ones that register
between 10-6 and 10-5 Watts per square
meter in the 1 to 8 Angstrom X-ray band on NOAA's Earth-orbiting
GOES 8 satellite. They are considered to be small compared to
the much larger M-class
and X-class flares that can erupt from active regions with
more complicated magnetic fields.

The "Bilbo Baggins" CME on January 31 was located near
a larger sunspot group (8841) composed of 8 tiny spots. That
active region also manifested a beta-type field, but the NOAA
Space Environment Center did not report an associated solar flare.
The January 31 CME is classified as a "disappearing solar
filament" (DSF) type, which means that it occurred at about
the same time that a filament rose off the Sun's surface. Filaments
are dense clouds of material suspended above the surface of the
Sun by loops of magnetic field. They can remain in a quiescent
state for weeks. As the magnetic loops that support them slowly
change, they can erupt and rise off of the Sun in just a few
minutes. A filament viewed in profile over the limb of the sun
is called a prominence.

"Although we understand the basics of why CMEs happen,"
continued Hathaway, "the details are still unclear. What
makes the fields unstable? How rapid is the onset of the explosion?
What's the detailed relationship between flares and filaments
and CMEs? All these questions are being actively researched,
and we still can't predict CME events with any reasonable degree
of accuracy."

With solar maximum slated for mid-2000 solar observers should
have plenty of opportunity to study solar flares and CMEs, and
to hone their space weather forecasting skills.

Parents and Educators: Please visit
Thursday's Classroom
for lesson plans and activities related to this story.

For more information about space weather and current solar
activity, please see SpaceWeather.com.
Technical information about current space weather condition may
be found at the NOAA Space
Environment Center. SOHO (the Solar and Heliospheric Observatory)
is a mission of international cooperation between NASA and the
European Space Agency. It is managed by the Goddard Space Flight
Center for the NASA HQ office of Space Science.